The predominant interests in the design of small, high-capacity hard disk drives include continuing to increase the total drive storage capacity while further reducing the overall size of the hard disk drive. As a consequence, the diameter of the hard disk media utilized to store data has decreased while the data bit density within the data tracks has increased.
Given that data is stored in a data band consisting of concentric data tracks subdivided into sectors of equal radial arc and equal data capacity, data sectors located on the inner diameter tracks must be recorded with a greater bit density relative to the outer diameter tracks. The read data channel must necessarily handle the full range of data frequencies read from the disk media. However, read channel errors will occur in response to timing variations and signal strength losses in the read data. These errors are most prevalent when reading data from the inner tracks.
Timing variations are due, in part, to a phenomenon known as bit crowding. Ideally, data bits are read as a signal peak generally centered within respective, fixed timing windows. However, the timing windows are continuously adjusted based on the period of the read data peaks. Bit crowding effectively shifts the data peaks, under certain circumstances, as read from the disk media. Not only are the affected peaks not centered within their respective bit windows, but the timing for subsequent timing windows is affected. As would be expected, any failure in detecting a read data peak appropriately within its correct timing window results in a read data error.
The bit crowding phenomenon is a consequence of the non-discrete reading of the data bits by a read head having a finite length perpendicular to the track width. Consequently, the read data signal strength is dependant not only on the characteristics of the data bit closest to the read head, but also on the adjacent bit characteristics. Specifically, sequential data bits of complementary polarity will destructively interfere. Thus, the peak signal magnitude read for each of a sequence of complementary data bits will be reduced. Where a peak is asymmetrically reduced, ie., the first or last bit in a sequence of complementary bits, the effective location of that bit peak is effectively shifted away from the nearest complementary bit. Thus, the noise margin is reduced due to the reduced peak signal strength of the read data and, further, timing variations are introduced. The peaks of middle bits are narrowed and reduced in magnitude resulting in a greater difficulty in detecting the narrowed peak and, again, a reduced noise margin. Although the resultant read data failures are in general "soft" errors, i.e., recoverable and intermittent, any increase in even the soft error rate leads to a proportional decrease in the performance of the disk drive as a whole.
Conventionally, the reduction in noise margin due to bit crowding has not been considered to be significant. Rather, a technique known as precompensation has been developed to reduce the significant timing variation aspect of bit crowding. Precompensation adjusts the write timing of data bits that would be asymmetrically shifted by bit crowding so that the data bit signal peak is read properly centered within its bit window. Conventional precompensation utilizes a fixed write timing offset for affected data bits for all data tracks less than a fixed, predetermined track diameter.
Another consequence of bit crowding is the roll-off in the amplitude of the read data as bit density increases. For any given data pattern, the density of data bits, as represented by flux changes, increases as the circumference of the data track decreases. Dependent on the frequency response of the read head, the active gap length and flying height of the head, the thickness of the media and other related factors, a data density threshold will exist above which the amplitude of all data bits will begin to reduce with increasing data bit density.
The conventional solutions to this problem include increasing the frequency response of the head, reducing the flying height of the head and decreasing the gap length of the head. Other solutions include using a high frequency peaking amplifier to enhance, or equalize, the sensitivity of the read channel while reading high data bit density data streams. Conventionally, an equalization amplifier having an increased gain factor above a fixed frequency threshold is provided in the read data channel. However, this latter solution is not desired for at least two reasons. First, the requisite peaking amplifier and delay line would add a significant design complexity and cost factor to the overall design and construction of the disk drive. Second, conventional delay lines suitable for use in such applications typically require a significant additional amount of driving power; a result that is not desired in the current and anticipated future generations of small, high performance hard disk drives.